Myogenesis in vitro has been much studied as a model of cellular differentiation. The formation of multinucleated non-proliferating electrically active and contractile myotubes by fusion of undifferentiated, proliferating myoblasts is accompanied by the appearance of muscle specific proteins, such as creatine kinase and myosin, and by the arrest of DNA synthesis. By manipulating the environmental conditions of a cloned line of myoblasts, I have been able to study these morphological and biochemical events. In addition, I have elucidated the nature of one of the cues that signal the cell to proceed along the path of differentiation rather than the path of continued proliferation, namely the concentration of the serum component of the nutrient medium.